In an IP telecommunications network, in which exchanges of signaling are based on the Session Initiation Protocol (SIP), for example, a user is identified by an address of record (AOR). This user may use this AOR on one or more terminal equipments, which behave as SIP user agents (UA) adapted to receive signaling messages sent to the AOR of the user. To be contactable at their AOR, a user must register their terminal in the telecommunications network beforehand, by initiating a registration procedure with a registration server or Registrar. Such a procedure has the effect of associating with the AOR of the user one or more addresses of contact (AOC) to which a signaling message, for example a call set-up request message, may be directly routed.
Such an association is stored in an SIP Location service database or server. Such a server is interrogated by a proxy server on reception of a signaling request sent to the AOR of the user with a view to obtaining the address or addresses of contact associated with that AOR.
There are at least three situations in which an AOR may be associated with a plurality of AOCs:                when the user uses their AOR simultaneously on a plurality of terminal equipments; for example, the user has a fixed terminal and a mobile terminal from which they have registered in the telecommunications network with the same address of record of the type alice@orange.com;        when the user uses a dual-mode or multi-mode terminal, i.e. a terminal that can be connected simultaneously to a plurality of access networks to the telecommunications network; for example, the user has a dual-mode terminal adapted to be connected to a Wi-Fi access network and to a UMTS (Universal Mobile Telecommunications System) access network simultaneously; and        a combination of the previous two situations.        
Such situations cause problems with routing a signaling request sent to the AOR of the user. According to the document RFC (Request For Comment) 3261 of the IETF (Internet Engineering Task Force) standardization organization, the proxy server is responsible for routing such a signaling request to one or more addresses of contact that form a target set. In particular, if the target set contains a plurality of addresses, the proxy server sends the signaling request to each of those targets in parallel or sequentially. This is referred to as forking.
There are prior art methods of limiting or controlling forking. In particular, the IETF document RFC 3841 teaches how to enable a proxy server to eliminate some AOC from a target set in order to adapt to the preferences of a calling user and to the capabilities of the terminal of a called user.
A drawback of such a mechanism is not allowing a choice of a particular type of access network to which to route signaling request.